Currently available cryogenic vacuum pumps (cryopumps) generally follow a common design concept. A low temperature array, usually operating in the range of 4 to 25 K, is the primary pumping surface. This surface is surrounded by a higher temperature radiation shield, usually operated in the temperature range of 60 to 130 K. The radiation shield protects the lower temperature array from radiated heat. The radiation shield generally includes a housing which is closed except at an opening where a frontal array is positioned between the primary pumping surface and a work chamber to be evacuated.
During operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the lower temperature array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the colder array may also be provided in this volume to remove the very low boiling point gases such as hydrogen. With the gases thus condensed and/or adsorbed onto the pumping surfaces, a vacuum is created in the work chamber.
In systems cooled by closed-cycle cryocoolers, the cooler is typically a two-stage refrigerator having a cold finger which extends through the rear or side of the radiation shield. High pressure helium refrigerant is generally delivered to the refrigerator through high pressure lines from a compressor assembly. Electrical power to a displacer drive motor in the cooler is usually also delivered through the compressor or a controller assembly.
The radiation shield is connected to a heat sink, or cold station, at the coldest end of the first stage of the refrigerator. The shield surrounds the second stage cryopanel in such a way as to protect it from radiant heat. The frontal array is cooled by the first stage heat sink through its attachment to the radiation shield or, as disclosed in U.S. Pat. No. 4,356,701, through thermal struts.
The coldest end of the second, coldest stage of the cryocooler is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at this coldest end of the second stage. This cryopanel may be a simple metal plate or cup, or it may be an array of metal baffles arranged around and connected to the second-stage heat sink. This second stage cryopanel also supports the low temperature adsorbent.
As part of the sophisticated technology employed to produce the utmost dependability and the highest efficiency of cryopumps, much effort has been devoted to the selection of materials for the regenerative heat exchangers in cryogenic refrigerators such as Gifford-McMahon, Stirling, and pulse tube cryogenic refrigerators. Regenerative heat exchangers which exhibit high volumetric heat capacities at low temperatures are normally preferred. As shown in FIG. 1, most metals, however, exhibit a sharp decrease in volumetric heat capacity with decreasing temperature below 75 K, in contrast with helium, whose volumetric heat capacity increases sharply below 25 K, peaking at approximately 10 K. The specific heat values shown in FIG. 1 for tin, antimony, helium, and lead are obtained from reference data, as disclosed in Thermophysical Properties of Matter: Specific Heat: Metallic Elements and Alloys, Y. S. Touloukian and E. H. Buyco, Vol. 4, and Specific Heat: Nonmetallic Liquids and Gases, Y. S. Touloukian and T. Makita, Vol. 6 (IFI/Plenum, New York 1970), the entire teachings of which are incorporated herein by reference. The specific heat values shown in FIG. 1 for mixtures of two or more metals are calculated by adjusting the known specific heat values of the pure metals by the percent composition in the indicated mixtures. Cryogenic refrigerators typically use lead (Pb) as a component of the second stage regenerative heat exchanger, because lead has a relatively high volumetric heat capacity at cryogenic temperatures.
Lead, however, is a poisonous metal that can damage nervous systems, especially in young children, and cause blood and brain disorders. Long term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO2) can cause nephropathy, and colic-like abdominal pains. Therefore, the use of lead in products is now either banned, restricted or undesirable.
Other regenerative materials, too, have disadvantages. For example, rare-earth containing intermetallic compounds are extremely expensive. In addition, intermetallic materials are harder and more brittle than metal compounds, and, therefore, are difficult to produce in the geometries needed for the regenerative heat exchangers in cryogenic refrigerators. These materials also have relatively poor performance because they can easily disintegrate into powder when exposed to repeated mechanical shocks during normal refrigerator operation. Bismuth is another metal with high volumetric heat capacity, but it is very expensive, brittle, and difficult to fabricate into the spherical shape needed for regenerator material. Bismuth can also disintegrate into powder like the intermetallic compounds, with the added disadvantage that bismuth powder is highly flammable and reactive with aluminum and air. Aluminum is a common material of construction in cryogenic refrigerators and thus the powder may react when the refrigerator is disassembled in air.
As such, there is a need for less hazardous and inexpensive regenerative heat exchanger materials with high volumetric heat capacity that don't have the potential to degrade over time during operation and are able to be formed into the required geometry.